The invention relates to an F2 laser, and particularly to an apparatus for monitoring one or more parameters such as the output energy of a laser beam of the F2 laser.
The present invention relates to the field of industrial molecular fluorine (F2) lasers and the application of F2 lasers in optical lithography, annealing, micro machining, photo-ablation and others. Excimer lasers currently used for these applications are mainly XeCl lasers (308 nm), KrF lasers (248 nm), and ArF lasers (193 mn).
In the future, F2 lasers (157 nm) will be more widely used based on their very short wavelength, particularly for such applications as sub-0.18 micron photolithography. This short wavelength, or high energy (157 nm=around 7.9 eV), photon emission is advantageous for photolithography applications because the critical dimension (CD), which represents the smallest resolvable feature size producible using photolithography, is proportional to the wavelength. This permits smaller and faster microprocessors and larger capacity DRAMs in a smaller package. The 7.9 eV photon is also readily absorbed in high band gap materials like quartz, synthetic quartz (SiO2), Teflon (PTFE), and silicone, among others, such that the F2-laser has great potential in a wide variety of material processing applications.
The construction and electrical excitation of the F2-laser differs from that of another type of gas discharge laser known as the excimer laser, referred to above. One difference is that the laser gas of an excimer laser includes a laser active constituent gas that has no bound ground state, or at most a weakly bound ground state. The laser active gas molecule of the excimer laser disassociates into its constituent atomic components upon optical transition from an upper to a lower state. In contrast, the laser active gas constituent molecule (F2) of the F2-laser responsible for the emission around 157 nm is bound and stable in the ground state. In this case, the F2 molecule does not disassociate after making its optical transition from the upper to the lower state.
The F2-laser has been known since around 1977 [see, e.g., Rice et al., VUV Emissions from Mixtures of F2 and the Noble Gases-A Molecular F2 laser at 1575 angstroms, Applied Physics Letters, Vol. 31, No. 1, Jul. 1, 1977]. However, previous F2 lasers have been known to exhibit relatively low gains and short gas lifetimes. Other parameters such as the pulse-to-pulse stability and laser tube lifetimes have been unsatisfactory. In addition, oxygen, water and other molecules exhibit high absorption cross sections around the desired 157 nm emission line of the F2-laser, further reducing overall efficiency at the wafer when encountered by the laser beam anywhere along its path. To prevent this absorption, one can maintain a purged or evacuated beam path for the F2-laser free or relatively free of oxygen and water (see U.S. patent application Ser. No. 09/343,333, assigned to the same assigned as the present application and hereby incorporated by reference). In short, despite the desirability of using short emission wavelengths for photolithography, F2-lasers have seen very little practical industrial application to date. Significant improvements are being made in the development of the F2 laser to achieve an F2-laser with enhanced gain, longer pulse lengths and pulse-to-pulse stability, and increased lifetime. Some of these improvements are described in the ""333 application mentioned above, and others at U.S. patent applications Ser. Nos. 09/317,526, 09/317,527, 09/317,695, and 09/598,522, each of which is assigned to the same assignee and is hereby incorporated by reference into the present application.
For many industrial and laboratory applications, excimer and molecular fluorine (F2) lasers are used in an operating mode wherein active stabilization of the output power of the laser to a preset, configurable value of power or energy is important. The active stabilization of the output energy of these lasers typically involves an energy detector indirectly connected to a control component of the driving high voltage of the discharge in a feedback loop and to a gas control system and, accordingly, actively adjusting the driving voltage and controlling gas injections and/or gas replenishments to stabilize the energy. This is possible because, as the output energy or output power of the excimer or molecular fluorine laser is selected to be maintained in a certain range, it is known that this output energy value depends on the input high voltage and the precise gas mixture in the laser tube. Thus, a variation of output energy may be compensated by adjusting the high voltage and laser gas mixture. See U.S. patent applications Ser. Nos. 09/379,034, 09/418,052, 09/343/3333 and 09/594/892 (describing techniques for compensating output energy variation based on halogen depletion including gas replenishment, as well as high voltage adjustments over limited voltage ranges), each of which is assigned to the same assignee as the present invention and which is hereby incorporated by reference into the present application.
There are other parameters of the output beam of an excimer or molecular fluorine (F2) laser that it is desired to monitor for various reasons. Among these output beam parameters are beam profile, bandwidth, wavelength, energy stability, pulse shape and pulse duration. In particular, it is desired to monitor any of these parameters in order to provide a feedback mechanism for controlling them during operation of the laser, particularly when the output beam is being used for precise industrial processing applications such as photolithography of small structures.
The VUV laser radiation around 157 nm of the F2-molecule has been observed as being accompanied by further laser radiation output in the red region of the visible spectrum. This visible light originates from the excited fluorine atom (atomic transition). It is desired to have an F2-laser wherein the parameters of the VUV (157 nm) portion of the output beam, and particularly the energy, may be monitored without substantial interference due to the accompanying red emission spectrum of the laser.
It is therefore an object of the invention to provide a F2 laser having an energy or other beam parameter monitoring unit wherein the VUV portion of the beam may be monitored without substantial interference from the red portion of the overall F2 laser emission spectrum.
The present invention is a beam parameter monitoring unit for use with a molecular fluorine (F2) laser that produces an output beam which includes ultra violet (UV) radiation and red radiation. The unit includes a mirror disposed to receive at least a portion of an output beam from the molecular fluorine laser, and a detector that measures at least one optical parameter of the output beam portion reflected by the mirror. The mirror substantially reflects ultra violet radiation in the output beam portion and substantially transmits red radiation in the output beam portion.
In another aspect of the present invention, a laser system includes a molecular fluorine (F2) gain medium disposed in a resonant cavity, a power supply for exciting the gain medium to produce an output beam having an ultra violet (UV) radiation output at substantially 157 nm and a red radiation output in a 620 to 760 nm wavelength range, a mirror disposed to receive at least a portion of the output beam, and a detector for measuring at least one optical parameter of the output beam portion reflected by the mirror. The mirror substantially reflects the UV radiation output in the output beam portion and substantially transmits the red radiation in the output beam portion.
Other objects and features of the present invention will become apparent by a review of the specification, claims and appended FIGS.